A combination weigher is the machine on a wet line a crew is most tempted to hose like everything else — and the one most likely to get wrecked doing it. The load-cell electronics and cable entry sit behind a stainless skin that looks washdown-proof but is only ever rated to its weakest neighbouring component.
Whether high-pressure hot water cleans the machine or drives water onto the load-cell board depends on three things the “mild detergent, don’t soak it” advice never gives you. The IP rating decides whether you can hose it; the temperature and pressure window sets how hard; and the sanitizer chemistry decides whether the steel passivates or pits.
The IP rating on the data plate is the first thing to verify, because “stainless” describes the metal, not the seal — and washdown failures are sealing failures.
IP69K is a specific test, not a marketing word: 80°C (176°F) water at 80–100 bar (1,160–1,450 psi) from a flat-fan nozzle at 100–150 mm, four angles, 30 seconds each. That is the washdown gold standard, run by USDA-inspected meat, poultry, and dairy plants.
IP65 is sealed against low-pressure jets only — aim a 1,400-psi lance at it and you are past what it survives. Higher number is not automatically safer, either: IP67 tests timed immersion and IP69K tests close-range jets, so verify the rating that matches how your crew actually cleans.
The real ingress path during washdown is rarely the weigher body; it is the lowest-rated component beside it. An IP65 indicator, junction box, or cable gland next to an IP69K scale becomes the leak point the moment the lance passes over it.
So the question is not “is my weigher washdown-rated?” but “is everything I am about to hose washdown-rated?”
If the answer is no, drop the lance settings to what the weakest component tolerates, or specify the gap out at purchase. A washdown-rated weigher built to IP65/IP69K with a sealed cable entry — the SC-200PH is one example — lets a crew run a lance over the machine without driving water past the gland.
Hot water cleans because heat breaks down fat and protein, so a washdown runs water at least 60°C (140°F) to melt organic soil off the surfaces, with high-pressure jets for the stubborn residue. General food-equipment washdown sits up to around 1,000 psi; the full 80°C / 1,450-psi envelope is for genuinely IP69K-rated equipment only.
The ceiling is set by your IP rating, not by how dirty the machine is: run the lance at whatever your weakest verified component is rated for, and no higher.
Thermal shock is the failure the temperature number hides. Drive 80°C water onto a cold weigh bucket or chilled load cell and the rapid expansion stresses the seals, then pulls moisture past a gasket as the part cools and contracts. Bring the machine up toward wash temperature gradually rather than blasting a cold unit.
Chloride is the number that turns “use mild chemicals” into a real limit: as little as 25 ppm begins to pit 304 stainless steel. Chloride ions break down the passive chromium-oxide film that makes stainless “stainless,” and once it is breached the pit drills beneath the surface where you cannot see it.
Because chlorinated sanitizers and municipal water are everywhere in a wet plant, a 304 weigher pits a little with every chlorine-based cycle.
316 stainless is the answer, and the reason is one element: 2–3% molybdenum. That molybdenum holds the passive film together in chloride-laden service and resists the pitting that wrecks 304, the same reason it is specified for CIP systems. Before you standardize a sanitizer, confirm which grade your weigher’s contact surfaces are.
Keep the chemistry off the seals as much as the steel. Strongly acidic and alkaline cleaners attack gaskets and cable-gland elastomers faster than the metal, and a degraded seal is the next ingress path. Match the cleaner to the steel grade and seal material, and rinse it off rather than letting it sit.
Wash from the top down so soil and runoff flow away from the most ingress-sensitive parts, never toward them. The load cell is the most vulnerable component — stainless and hermetically sealed at best — and the part a lance must never be aimed at directly.
Start at the top frame and weigh heads, work down through the buckets and chutes, and finish at the base, so dirty water runs out rather than pooling on a junction box. Water driven directly past a cable-gland seal or load-cell junction box pools on the electronics, and that is what later reads as drift.
Keep the lance off three points and check their O-rings on a schedule: motor housings, load-cell junction boxes, and cable entries. These seals fail quietly, and a cracked O-ring on a junction box turns a routine washdown into a short.
Tubular members, blind sockets, and socket-head cap screws are harborage points you cannot swab. Open-frame, fully drainable construction with standard fasteners is what makes a weigher genuinely cleanable. A machine you cannot inspect is unverified, not clean — which is exactly what 21 CFR Part 117, EHEDG, and 3-A require you to design out.
Dry the machine thoroughly before it returns to service, because drying is corrosion control, not housekeeping. Chloride does its worst damage on a drying surface: as water evaporates, the chloride concentrates, and that film keeps pitting the steel long after the rinse water is gone. A weigher left to air-dry with chlorinated residue corrodes faster than one wiped down.
Drying also closes out the accuracy problem. Water past a seal, plus residual film on a load cell, produce post-wash drift and false rejects when production resumes. The weigher reads off because it is wet or thermally shocked, not because the load cell failed.
Before the line restarts, dry the contact surfaces and verify the readings with certified weights per the manufacturer’s procedure. That catches drift before it shows up as a batch of underweight or over-rejected packs.
If the weigher still reads off after a correct washdown, the cause is upstream water ingress or thermal shock — see how to troubleshoot a multihead weigher that reads inaccurate after cleaning.
Washdown frequency is set by product risk, not a fixed calendar — the higher the contamination or allergen risk, the more frequent the washdown and the higher the pressure it justifies. A dry-snack line on one non-allergen product can stretch its wet washdowns; a meat, poultry, or dairy line, or any allergen changeover, needs a full cycle far more often.
Allergen changeover is where washdown is not optional. Wiping and blowdown only move residue around. A wet washdown is what physically flushes it off the buckets and chutes, which is why a line handling allergen cross-contamination on a multihead weigher schedules wet cleaning around every product change.
High-care verticals are where the IP-rating and frequency decisions converge. Meat, poultry, and dairy plants wash often and hard, at the 80°C / 1,450-psi exposure that demands an IP69K machine.
A weigher washing daily under a lance has to be specified for that duty from the start, because the frequency is what wears down an under-rated seal. The general sequence this washdown sits inside is the guide on how to clean a multihead weigher.
The mistake that wrecks more weighers than any chemical is treating the rating as a property of the scale rather than the whole assembly. Verify the IP rating of every component you are about to wet — not just the weigher body — and let the weakest set your pressure, temperature, and aim.
The corrosion side is two numbers and one discipline: 25 ppm of chloride starts pitting 304, 316’s molybdenum survives it, and an air-dried surface concentrates whatever chloride is on it. Match the sanitizer to the steel grade, then dry the machine as deliberately as you washed it.
A weigher washed hard and dried thoroughly holds its accuracy across a season once certified weights confirm it before restart. The one hosed like the rest of the line and left to drip is your next false-reject investigation.
